When they are transferred and stored, the audiovisual documents are encoded and therefore compressed. The purpose of compression is to reduce the size of the document so that it can be sent more rapidly and so that it occupies less space when stored. These days, there are numerous video, audio or audio/video encoding algorithms. Those relating to video include MPEG2, MPEG4, H263, H264, etc. The encoding algorithms differ according to characteristics such as: their compression ratio which determines the size of the document, the computation capability required, the speed of encoding and decoding, the quality obtained on playing back the compressed document, and so on. Depending on the type of medium used to store the work, the algorithm and the encoding parameters are not the same. At the time of playback, the playback device reads the encoding parameters of the document in order to determine which decoding algorithm to use. If the device does not have the decoding means, it returns an error message to the user.
In a home network context, the playback devices are linked to devices for reading and/or storing audiovisual documents. Communication within the network is conducted via cables or wirelessly, but in both cases, a protocol known to all the devices enables each message sent to be received and processed by the right recipient. These devices are used in the multimedia field, for example: a television receiver, a video recorder, a DVD player, a decoder, an amplifier, an audio CD player, and so on. Some devices have a means of communicating with an external network, such as the Internet, via an interface linked to the telephone network or a high speed network. The network devices are normally made by different manufacturers and therefore have incompatible encoding and decoding algorithms. Because of this, the choice of an audiovisual or audio document is based on the type of decoding algorithm that can be executed by the playback device.
A network commonly incorporates a number of playback devices. For example, each room in the home has a terminal. A number of receivers receive documents from a number of transmission networks. The communication network has a number of storage devices of different types, such as a digital video recorder and a hard disk. Network management can be handled centrally using a control device or be distributed, with each device having the capability to control the network in turn.
Documents intended for display have attributes that enable them to be identified. The identifier of a broadcast is typically its title and its transmission channel, this information being displayed via an electronic program guide (EPG). The user can select a broadcast and watch it on a screen. Systems in which a user can move to another room and watch the same broadcast on another screen are known. For this, the user has a personal electronic badge. This badge is connected to the terminal that the user is currently using, for example to watch a broadcast transmitted on a particular television channel. When the user leaves a terminal, he removes his badge and the latter stores the current task. Then, when the user starts a session with a new terminal, he connects his badge which automatically transmits the characteristics of the last task requested by this user, so the new terminal displays the same broadcast as previously.
This “follow-me” application enables a user to have the playback of a particular document follow him as he moves from one device to another within a network. However, it is essential for all the devices to have the decoding algorithm corresponding to the encoding of the document. If a device does not have this algorithm, it cannot display it and therefore the screen appears black to the user. One of the advantages of the present invention is to overcome this problem by making it possible for any playback device selected by the user to be able to display the audiovisual document. The “follow-me” application is in particular described in international patent application WO03/067594 filed by UCENTRIC Holding, Inc.
Patent application U.S. 2003/0066084 published on 3 Apr. 2003 relates to a system for transcoding a document passing through a network. A first device stores a document in a first format, then a second device transcodes the document in a second format to make it compatible with the playback unit.